1. Field of the Invention
The present invention relates generally to vehicle charging systems, and more particularly to a charging system having a half-wave or a full-wave controlled rectifier bridge and a single voltage sensor.
2. Background
A typical prior art charging circuit 10 is depicted in FIG. 1. The charging system provides electrical energy while the engine is running to recharge the battery and to power electrical devices. As shown in FIG. 1, a battery 12 is connected between a ground 14 and a positive or "hot" lead 16, which leads to the electrical systems (not shown) of the vehicle and to the alternator 18. This lead 16 is a path for current out of the battery 12 during undercharging or discharging, and a path for current into the battery 12 during charging. The alternator 18 is typically driven by a pulley 20, which is driven by a belt (not shown) from the prime mover or engine (not shown). The electrical systems of the vehicle are powered through lead 22. An ignition switch 24 is also connected to the hot lead 16. Typically, an indicator lamp 26 is present to indicate a discharge state. A voltage regulator input lead 28 connects to the voltage regulator 30, which determines the output of the alternator 18 by controlling the excitation voltage provided to the field winding of the alternator via line 32, as will be discussed in greater detail below.
The basic layout of a vehicle alternator is well known. An alternator is typically a three-phase AC generator that typically comprises a rotor, which is essentially a spinning magnetic field which is turned by the vehicle's engine, and a stator, which is a stationary output winding. The operation is based on Faraday's law of electromagnetic induction. As the rotor is moved creating a varying magnetic field, electromotive force, or EMF, is induced in the windings resulting in current output. In order to produce a magnetic field in the rotor, field windings in the rotor are connected to a source of excitation current. The output from each of the three phases is AC, which is then rectified into DC through a rectifier bridge.
A prior art rectifier bridge is depicted in FIG. 2. Prior art rectifier bridges for vehicular alternators have typically used an arrangement of diodes D1-D6, which serve as electrical check valves. Each of the three phases V1-V3 is connected to two diodes, such that the negative and positive AC output of each phase are each rectified into DC voltage. FIG. 3 depicts the output that results from the prior art diode bridge, as is well known to those skilled in the art.
The prior art rectifier bridge is very dependable and has been used successfully in claw pole, synchronous (Lundell) alternators in vehicles for decades. However, the modern era has placed increasing power demands on vehicle electrical systems through the constant addition of new electrical and electronic accessories, control systems, etc. to the vehicle. It has also become increasingly less desirable to solve these power requirement problems through increasing the size of the alternator, as available space under the hood of the modern vehicle is densely packed. Further, the additional engine power required to turn a larger alternator decreases overall efficiency of the vehicle, as does the additional weight. Finally, overall cost is an extremely important criterion in evaluating vehicular design solutions, rendering a larger more costly alternator less desirable.
A result of the increasing demands placed on the alternator is that often when the engine is at an idle, when alternator speed and hence power output efficiency are quite low, a deficit in the charging system results. In such situations, the battery supplies the required energy in a discharging state. Repetitive charging and discharging of batteries used in vehicles, typically lead-acid storage batteries, leads to shorter longevity, which is undesirable both from the point of view of the consumer and the environment.
One limitation of the diode is that it is not controllable in the sense that its switching points are inherent in its design. The prior art contains other arrangements, which replace the diodes of a conventional rectifier or inverter bridge with controllable elements, such as transistors or thyristors, but satisfactory control regimens for these controllable elements have been difficult to achieve and have not been successfully implemented.
U.S. Pat. No. 4,489,323 discloses a three-phase alternator whose output is rectified by a full wave thyristor, or semiconductor-controlled rectifier (SCR), bridge. One phase output is measured to generate a square wave in order to determine the speed of the alternator, so that the full-wave rectification can be switched to half-wave rectification when the speed of the alternator becomes too fast for the SCRs to fire reliably for proper rectification. The disadvantage with SCRs is that they naturally commutate with zero crossings of fundamental voltage waveform, rendering them less amenable to control. This apparatus disadvantageously does not address the problems associated with underspeed operation, only those associated with overspeed operation and SCR switching reliability.
U.S. Pat. No. 5,648,705 discloses a system for improving the power output of a vehicular alternator at low speeds through the use of a controllable rectifier bridge. A current detector and a voltage detector measure the state of one phase of the three-phase stator winding, and this information on the state of the single phase is passed to a controller. The controller uses the information provided by the detectors to control a full-wave controlled rectifier bridge in order to optimize the phase shift between the back EMF in the stator winding and phase voltages at the three output connections of the stator winding. The functioning of the device defined in this patent in its basic phases can be understood starting with reference to FIG. 5 of the patent, which describes a prior art alternator device comprising a conventional diode rectifier. For a Lundell alternator such is commonly used in vehicles, the power angle .beta. represents the angle between the back EMF and the phase voltage (V). As can be seen on the bottom vector of FIG. 5, a conventional diode bridge has an angle of zero between the current (I) and voltage (V). The resulting angle .beta. is therefore less than 90 degrees, which results in a lower power output jIX.sub.s. It is accordingly desirable to increase the power angle .beta.. To avoid the use of rotation sensors, and because the EMF cannot be directly measured during loading of the alternator because it cannot be electromagnetically separated from the armature current (reaction), the '705 device and methodology measure the current and the voltage of a single phase of the stator winding. By these measures, the synchronous frequency of the alternator can be determined, which reveals certain information about the back EMF. This information can then be used to maintain a phase shift a between the phase current and its associated phase voltage to maximize output, as seen in FIG. 4. The phase shift can be induced by controlling the switching of the rectifier bridge. The control strategy employed uses a phase current detector and a phase voltage detector to estimate the position of the back EMF from the determined phase current and voltage. This estimated position provides an existing delay angle between the phase current and voltage, which is compared to a desired delay angle read from experimentally determined optimum values in a look-up table. The controllable bridge switching is then manipulated to match the estimated delay angle to the optimum angle. Disadvantageously, this control regimen requires the measuring of current and voltage, which requires two sensors and the electronics to gather and process the two data.
U.S. Pat. No. 5,793,167 discloses a system for increasing output power from an alternator, wherein a conventional full-wave diode bridge of the alternator is replaced with a full-wave controlled rectifier bridge having controlled switches in place of diodes, and the rectifier bridge is controlled in response to a third harmonic of the voltage generated by the alternator to synchronize the rectifier bridge with the alternator. The alternator includes a rotor having a field winding receiving a field current, which is controlled up to a maximum field current for partial control of the output power produced by the alternator. Power produced by the alternator is also controlled by introducing a phase angle between the phase voltages at the three output connections of the stator winding and the third harmonic up to a maximum or optimum phase angle. To increase power output from the alternator, the field current is increased up to a maximum before any phase angle is introduced between the phase voltages and the third harmonic. Similarly, the phase angle is reduced to zero before the field current is reduced if power generated by the alternator is to be decreased. Disadvantageously, this control technique also requires the measuring of current and voltage, thereby requiring two sensors and the electronics to gather and process the two data.
The voltage regulator, a major part of the charging system, controls the output of the alternator by controlling the excitation current in the field windings. By changing the excitation current in the field windings in the rotor, the strength of the magnetic field of the rotor is affected, and thus the output of the stator windings of the alternator. Prior art voltage regulators typically are preset to maintain the charging voltage of the alternator at a predetermined point, typically between 13 and 15 V. In an automotive charging system, in order for the battery to recharge, the output voltage of the alternator must be higher than that of the battery. However, a large difference can overload the battery, causing electrochemical damage, which decreases its longevity. For this reason, only a small potential difference above the typical 12 to 12.6 V of a fully charged battery is used. Because the rotational speed of the alternator varies with engine speed, the voltage regulator is necessary to maintain the voltage of the alternator output.
A prior art electronic voltage regulator is depicted in FIG. 6. By placing resistances in and out of series with the excitation field current (which is supplied in most cases by the battery), the strength of the excitation field in the rotor can be modified. In this embodiment, if the alternator speed is too low and/or electrical load too high, the regulator will compensate so that the alternator achieves a preset voltage output. Other systems vary the duty cycle of the field windings in order to arrive at a preset voltage output. During periods of low output, the duty cycle could be as low as 10%.